Carla Bruni, France's first lady, has vowed to help a grieving Italian mother find out what happened to her son, who died in mysterious circumstances in a French prison.

Italian-born Ms Bruni (43) was "deeply moved" after being contacted by Cira Antignano about the death of her son Daniele Franceschi.

Mr Franceschi (36) was arrested by French police in Cannes as he tried to use a credit card that did not belong to him in a casino in March and he died five months later after complaining to his mother of mistreatment.

Officially, French police say he died from "natural causes", but Mrs Antignano wants a full explanation from them about why his organs were missing when his body was returned.

In her letter, Ms Bruni said: "I was very deeply moved by your letter and I want to express my deep condolences. I want you to know that I am close to you in your pain.

"I have full faith in the French justice system and they will answer you and the Italian authorities. There is ongoing contact between them both. Be assured you have my solidarity, keep strong and brave."

Ms Antignano had written to her last month asking for help. She said: "I couldn't believe it when I got the reply from her. I hope my faith in French justice will return."

French authorities are investigating the death, which took place in Grasse prison, and have promised to return the missing organs by the end of next month so that a post-mortem on them can be carried out in Italy.

Meanwhile, cocaine with a street value of more than £6m (€7.05m) has been found at the flat of a royal neighbour of French President Nicolas Sarkozy in the Paris suburb of Neuilly-sur-Seine.

Armed officers arrested two men and two women following the raid, while two more suspects are on the run.